Project Summary Serotonergic and dopaminergic systems of the brain are broadly implicated in mental illness including depression and mood-related disorders. How these systems interface to modulate affective state in the presence of stress and recovery from stress remains poorly understood. Dopamine neurons located in the ventral tegmental area (VTA) and serotonin neurons located in the dorsal raphe nucleus (DRN) share common targets. Both project to the prefrontal cortex, nucleus accumbens, hippocampus, and amygdala. Each of these regions has been implicated in depression-related symptom domains and the balance of dopamine and serotonin in these regions is an important regulator homeostatic response to stress. Serotonin neurons of the DRN directly synapse onto dopamine neurons of the VTA, indicating a direct interaction between these systems. To establish the role of this circuit connection in behavioral regulation of stress and behavioral domains relevant to depression, we will perform a detailed circuit analysis of serotonergic inputs from the DRN to dopamine neurons of the VTA and how this connectivity changes in response to social defeat stress in susceptible and resistance mice. To enhance the granularity of our circuit analysis, we will measure excitatory and inhibitory coupling between serotonin and dopamine neurons in dopamine cells that project to specific areas of the brain. To ascertain how reward responding of serotonergic neurons change at the local circuit level of the DRN following social defeat in susceptible and resistant mice, we will monitor in vivo calcium dynamics in all serotonin producing neurons of the DRN and exclusively in those cells that synapse onto dopamine neurons. To establish the functional importance of serotonergic DRN input to the VTA in the mitigation of depression-related symptom domains following social defeat, we will selectively enhance the activity of DRN neurons that synapse onto dopamine cells using designer receptor (DREADD) technology. These studies, combined with the efforts of my collaborators in this center grant will provide an unparalleled assessment the neural circuitry underlying susceptibility and resistance to social defeat stress, elucidate the therapeutic potential of an endogenous opioid system and how it interfaces with these circuits, determine the impact of sex-specific differences in these circuits associated with stress-induced alterations in decision making, and provide detailed assessments of transcriptional and translational changes in treatment resistant females suffering from depression and in rodents susceptible to depression-related symptoms.